The present invention relates to pyrotechnic actuators, and, more specifically, to a pyrotechnic actuator that incorporates a tapered piston thereby simplifying manufacturing of the actuator while resolving performance concerns.
Pyrotechnic actuators are known. Cylinder-piston type actuators of a wide range of dimensions are utilized in a multitude of applications, whereby upon activation of the actuator, a load exerted on the actuator often causes the piston/actuator to retract from its activated position. In hydraulic actuators, the reactive load may be held by blocking return flow of the hydraulic fluid into the cylinder, as by means of an appropriate valve. The same is true of pneumatic actuators. Such actuators, however, are usually heavy and cumbersome and are not suitable for uses wherein a small, lightweight actuator is required. For example, automobile design requirements in certain countries require that pyrotechnic actuators function to raise the vehicle hood immediately upon impact with a pedestrian. By increasing the distance from the engine block to the underside of the hood, the point of impact is effectively softened given the additional space for mechanical deformation of the hood.
One solution to the piston retraction problem is the insertion of a latching mechanism that extends inwardly within the piston cylinder once the piston has been propelled forward upon combustion. Another solution comprises a split ring carried by the piston, expansively engaging the internal wall of the piston cylinder and expandible at the end of the piston stroke thereby holding the piston securely in its projected position. U.S. Pat. No. 4,091,621 is exemplary and is incorporated herein by reference. Although apparently effective, these approaches increase the cost and complexity of manufacturing
Another disadvantage in known pyrotechnic actuators is the low thermal stability of many gas generant compositions used therewith. As a result, known pyrotechnic actuators typically function over a small temperature range. Therefore, given the close proximity to the engine block, the gas generant composition employed in the actuator should preferably exhibit a relatively high autoignition temperature and a high thermal stability when cycled from xe2x88x9240xc2x0 C. to 140xc2x0 C.
The aforesaid problems and others are resolved by a pyrotechnic actuator that contains a tapered piston fixedly received by an annular retaining element upon activation of the actuator. In accordance with the present invention, the actuator contains a housing having a piston chamber formed longitudinally therein. A tapered piston is contained within the chamber. A gas generator positioned at a first end of the housing fluidly communicates with the piston upon activation of the actuator. The gas generator includes a pyrotechnic gas generant composition ignitably communicating with or proximate to an igniter also contained therein. Upon activation of the actuator, by an impact or crumple zone sensor for example, the gas generator produces gas and a resultant pressure thereby driving the piston. The tapered piston slidably engages an inner wall of the piston chamber and is propelled toward a second end of the housing. The tapered portion of the piston has a cross-sectional area that changes over its length and therefore is respectively increased in size from a point closer to the second end of the housing to a point closer to the first end of the housing. As a result, upon actuator operation the piston initially freely passes through an annular piston retainer fixed within the second end of the housing. As the cross-sectional area (a diameter for example) of the piston increases by virtue of the tapered piston portion, the piston is fixedly received or wedged within the annular retainer thereby preventing piston retraction when a load is applied to the piston.
At its largest diameter or cross-sectional area, the piston is roughly equivalent to the cross-sectional area of the chamber. Conversely, at its smallest cross-sectional area, the piston is roughly equivalent to or smaller in cross-sectional area as compared to the total area of the retainer annulus. As a consequence, the tapered piston enters and slidably engages the inner wall or annulus of the retainer until the piston is wedged tightly therein as the cross-sectional area or diameter of the piston exceeds that of the retainer annulus. Upon exertion of a load approximately normal to the top surface of the piston, retraction of the piston is thus prevented. The cross-sectional shapes of the chamber, piston, and retainer annulus are preferably the same thereby enhancing sealing and performance of the actuator. For example, if the chamber is formed with a circular cross-section, then the respective cross-sections of the piston and retainer annulus would also preferably have a circular cross-section or diameter.